Optically pure .alpha.-cyanohydrins are versatile synthetic intermediates, the two functional groups being easily manipulated into a wide range of other chiral products such as .alpha.-hydroxy acids, .alpha.-hydroxy aldehydes, .alpha.-hydroxy ketones, .beta.-hydroxy amines and .alpha.-amino acid derivatives etc.
The usual synthetic route to cyanohydrins was invented more than one hundred years ago. However, the cyanohydrin produced by this well known way is a racemic product. If optically active cyanohydrin is needed for further mulipuulations, additional resolution step(s) for the racemic cyanohydrin would be required. To overcome this problem, several asymmetrical syntheses of cyanohydrin have been developed. Most of these asymmetrical syntheses use a chiral catalyst to induce the formation of just one enantiomer of the cyanohydrin. So far, a number of different catalysts have been investigated, including enzymes, polymeric reagents, peptides and organometallic species. Among them, organometallic species are the catalysts developed recently.
Organometallic species, which were used as the catalysts to catalyze the asymmetrical addition of trimethylsilyl cyanide to aldehydes, were disclosed in several technical literatures. However, drawbacks could also be found among these organometallic species. For example, the organometallic complexes of binaphthaol and titanium tetraisopropoxide, which are reported by Nakai et al., have good enantioselectivity but only for aromatic aldehydes. The organometallic complexes reported by Jiang et al. have enatioselectivity but are easily deteriorated by acids. Bolm et al. also reported organometallic complexes with good enantioselectivity, but stoichiometric amounts of the organometallic complexes catalyst were required.